What is the significance of a Thyroid Hormone (T4) uptake of 28% in thyroid disease?

A T4 uptake of 28% indicates decreased thyroid hormone binding capacity in the blood, which is often seen in hyperthyroidism. This test measures the availability of binding sites on thyroid hormone transport proteins, primarily thyroxine-binding globulin (TBG). When the percentage is low (normal range typically 25-35%), it suggests that binding sites are already occupied by elevated thyroid hormones, as commonly occurs in hyperthyroidism. However, this test alone is not diagnostic and must be interpreted alongside other thyroid function tests like TSH, free T4, and free T3. If you have this result, your doctor will likely order these additional tests to confirm a diagnosis. Low T4 uptake can also occur with protein abnormalities or certain medications like androgens or glucocorticoids that affect binding proteins. Treatment depends on the complete thyroid profile and clinical symptoms, not on this single value. The T4 uptake test has largely been replaced by direct free thyroid hormone measurements in modern practice, but when used, it helps calculate the free thyroxine index (FTI), which provides a more accurate picture of thyroid function than total hormone levels alone.